Drawing constant current from a line



April 21, 1970 E. W/RUMMEL' 3,508,139

DRAWING CONSTANT CURRENT FROM A LINE Filed Match 14, 1967 I NVEN TOR.Edward W. Rummel .4 w WJQ United States Patent DRAWING CONSTANT CURRENTFROM A LINE Edward W. Rummel, Cleveland, Ohio, assignor to WesternReserve Electronics Inc., Cleveland, Ohio, a corporation of Ohio FiledMar. 14, 1967, Ser. No. 623,096

Int. Cl. G05f 1/20; H02m 7/18; H02j 13/00 US. Cl. 323-4 4 ClaimsABSTRACT OF THE DISCLOSURE A transistorized device connected across atelephone line to draw constant current therefrom-to enable testing ofthe line. Thedevice is connected to the line by a diode bridge havinglamps in the bridge arms to indicate line polarity.

This invention relates to electronic circuits which draw a predeterminedamount of current in the presence of a varying voltage. Morespecifically, it includes an electronic line holding circuit.

The invention will be described in conjunction with its use for holdingthe relays in a telephone line closed but is not necessarily limited inits use on telephone lines. As is well known to those versed in the art,it is frequently necessary to test the characteristics of a telephoneline, usually extending from a central or a repeater station to asubscriber, particularly for line losses. Such lines include relayshaving line closing contacts which are normally locked in when thetelephone is removed from the cradle. In its normal (non-use) positionno current is being drawn and the relays are open. When such a line isto be tested it is necessary for the relay contacts to be closed. Underthese (closed line) conditions a signal of a predetermined frequency,such as 1000 cycles, may 'be sent into the line and a measurement of theloss in decibels made at the other end of the line and thus the state ofthe line determined. In order to keep the relays closed it is commonpractice to remove the hand set and place a load on the line whichusually comprises an inductance in series with a resistance. Suchdevices cause certain difiiculties in that the inductances must be largeenough not to shunt the desired'signal and still enable the desiredcurrent to be drawn to keep the relays closed. Such a device is heavyand'materially increases the weight of the instrument which does themeasuring. .It .is also bulky and takes up a considerable amount ofspace. It is also expensive to manufacture. It

has many electrical properties which act to its detriment.

For instance, in an iron cored device, should the current through theinductor exceed a certain value, the core becomes saturated and thevalue of the inductance is no longer what it should be. Because of thevariation in line length the normal DC. voltage on the line terminatorwill vary making it necessary to provide inductances of different valuefor different lines.

The present invention contemplates a compact, light weight, inexpensivedevice which uses solid state components and will draw a constantcurrent of the desired amount when placed in a line having a voltagewhich may vary, and wherein its operating voltages are derived from theline. It is not polarity conscious and therefore operates regardless ofwhich of its terminals are connected to the line. Indicator means may beprovided which will indicate the polarity of the lines if it should bedesired.

Its more specific use is for terminating a telephone line and drawingsufficient current to cause the relays to lock in, whereby a signal maybe applied to the line without any material shunting effect on thesignal.

The figure of the drawing illustrates diagrammatically 3,508,139Patented. Apr. 21, 1970 the device of the invention and its manner ofconnection for use as a termination for, and to lock in the relays of atelephone line. I

As shown in the drawings, line L and L connect to a remote station, notshown, but wherein the holding relay contacts 10 are showndiagrammatically. Normally the line may be terminated at one end by theusual telephone 12. In this arrangement when the telephone is in thecradle, the only thing on the line is the ringing coils, therefore theline relays are open. The telephone may be removed, when it is desiredto test the line, and a 1000 cycle signal applied to the line beyond theholding contacts at 11. The problem, then, is to connect between thelines, a device which will draw a current, the same as the telephonewould when the hand set is removed from the cradle, to cause the relays10 to lock in and complete the circuit without materially affecting thecircuit from the testing standpoint.

The outer portion of the circuit comprises four legs, two adjacent legson one side being connected to one side of the line and the two adjacentlegs on the opposite side connected to the other line. Diodes areprovided in the legs, those, D in the first set of adjacent legs beingconnected in the same directions and their-junctions connected to line LThe diodes. D in the other second or opposite set of legs are also insame direction with each other but in opposite direction to the otherdiodes and their junctions are connected to the second line L It will beapparent that with this configuration no substantial current can passthrough the legs because, with a positive voltage applied at L currentcannot flow down and must go through the upper diode D At the other sideit is blocked by the upper diode D in the upper adjacent leg. Likewisean A.C. signal of a frequency useful to test the lines would not passthrough the legs.

The constant current drawing means is connected across the bridge orbetween the junctions 14-15 of the opposite legs, and includes a PNPtransistor T an NPN transistor T and an NPN transistor T and theassociated circuitry. The transistor T is the main current drawingtransistor and in conjunction with the other (controlling) transistorsdraws the desired current in the presence of varying D.C. line voltagesdue to varying line resistance. Transistor T is a voltage amplifier, theoutput of which controls T and T provides the bias for T which biaschanges with the line voltage and thus maintains the current through Tconstant and of a value to lock in the relays. It will be apparent tothose versed in the art that the type of transisors could be reversed.

The emitter of T is conected to the bridge diagonal at 14 and thecollector is connected by two branches to the other end of the bridge,the right branch being comprised of two diodes D and D in series witheach other and a resistor R to the other junction 15, which provides afinite voltage drop and the operating voltage for T The left branchincludes the resistor R which is connected to the collector of an NPNtransistor T the emitter of which connects to the junction 15. Thuscurrent may flow through this transistor which also operates as avoltage amplifier. The current through transistor T splits equally inthe two branches and is combined at 15 and passed up through the diode Dto the line L The output of T is connected to and controls T thecollector being connected to the base of T The collector of T connectsto the base of T The emitter of T connects to the junction of D and R Tis thus the control for T and couples T to T Resistor R connects betweenthe point 14 and the base of T providing a voltage feed-back injectingmore base current into T should the line voltage increase and causeincreased heat in T This increases the fixed bias to T 3 and causes adecrease in fixed bias on T and T R isolates the base of T from theemitter of T for the purpose of the voltage feed-back.

When the device is connected across the line L and L after the telephoneis removed from the circuit, the device operates to draw current, whichmay be 20 ma., and to close the contacts 10. A signal may now beinjected into the line and the testing proceed in the usual manner. Itis apparent that the parameters of the circuit may be adjusted to causethe device to draw the amount of current necessary to lock in the relaycontacts and this may be other than 20 ma.

Assuming the line L to be the positive side of the line, conventionalcurrent flow will be upward through the diode D to the point 14. It isblocked by the diode D in the adjacent top leg and therefore travelsdown through the transistor T through the emitter and out the collectorwhere it divides in equal parts, one through the right branch throughthe diodes D and D and resistor R and the other through the resistor Rand the transistor T The current through the two branches combines atthe junction 15 of the lower legs and then passes through the lower leftdiode D to the line L The collector of T is more negative than theemitter because it receives negative voltage from the line from lowerdiode D through the right branch R D and D and through the left branch Tand R The base current from T is through transistor T Diodes D and Dprovide a finite voltage drop which provides the operating voltage forthe voltage amplifier T T is responsive to variations in line voltages,due largely to differences in line length; its output will vary withline voltage. It will have higher output current, with higher voltageand less collector to emitter voltage which is applied to the base Twhich reduces the current in T The output of T being connected to thebase of T as the line voltage goes up, the base bias on T decreases andthe current through T will stay the same. As the voltage goes up,tending to make the main current through T go up, the increase involtage drop across R increases the bias to T This causes the collectorcurrent to T to increase. The top of R being connected to the emitter ofT the voltage on the base goes down when the line potential rises. Thedifference between the voltage on the base and emitter, going down,causes the current in the base to go down and along with it thecollector current, in direct proportion. This is the new current that isnecessary to keep the current through T constant.

Current feedback is realized by the IR drops in R and R The top R goesto emitter of T and the bottom to the emitter of T A secondary voltagefeedback is provided from D through R to base of T injecting more basecurrent into T R decouples the base of T; from the emitter of T foreffecting this voltage feedback. By maintaining a constant currentthrough the transistor T in the presence of line voltage changes thedynamic impedance is substantially infinite and therefore one of thetest signal current is conducted through the bridge.

It will be apparent that the lamps 23 and 24 may be inserted in the legsand will be selectively energized depending upon through which leg thecurrent from the lines is flowing. If L is positive and current flowsthrough upper diode D the lamp 24 will be energized. If L was positivethe flow would be from L through the upper diode D and lamp 23 would beenergized. Thus the polarity of the line is indicated. Regardless of thepolarity of the individual lines L and L the current flow is always intothe top of the bridge 14 which thus becomes positive with the bottom 15being negative. Therefore,

the device is not conscious of line polarity.

I claim:

1. A device for connection across a line arranged to draw apredetermined constant amount of current in the presence of variationsin line voltage and receiving its operating potential from the linecomprising, a bridge having unilateral conductances in each leg andarranged toprevent the flow of current around the bridge and thejunctions of adjacent legs on opposite sides of the bridge beingarranged to be connected across the line, the other junctions of saidlegs having constant current drawing means connected directly theretoand extending across the bridge, said current drawing device including afirst transistor connected to pass current across the bridge, a secondtransistor in series with the first transistor and responsive to voltagevariation in the line and a third transistor is connected to andcontrolled by the second transistor and furnishes a bias to the firsttransistor and causes it to pass constant current.

2. A device as decsribed in claim 1, wherein part of the current fromthe first transistor passes through the second transistor and circuitmeans is provided connected to the first transistor and the other sideof the bridge for passing the remainder of the current, said last meansincluding a voltage dropping means connected at an intermediate point tothe third transistor.

' 3. A device as described in claim 2, wherein resistance means isprovided connected to the same end of the bridge as the first transistorand .to the base of the second transistor.

4. A device as described in claim 3, wherein the means for passing theremainder of the current includes a pair of serially connectedunilateral conductances and a resistance and said intermediate point isconnected to the base of the second transistor and the emitter of thethird transistor.

References Cited UNITED STATES PATENTS 3,161,759 12/1964 Gambill et al.3,201,754 8/1965 Reiner et al. 340-176 X 3,246,233 4/1966 Herz 323-43,375,434 3/1968 Shapiro 323-4 3,408,557 10/1968 Horgan et al. 323-1'FOREIGN PATENTS 130,916 12/ 1932 Austria.

LEE T. HIX, Primary Examiner A. D. PELLINEN, Assistant Examiner US. Cl.X.R.

